1. Field of the Invention
The present invention relates to an image forming apparatus for developing an electrostatic latent image to form a visualized image.
2. Related Background Art
It is practical that an image bearing member such as an electrophotographic is heated to a predetermined temperature (for example, about 40.degree. C.) by a heater disposed within the image bearing member in order to prevent the occurrence of image flow due to the adhesion (dewing) of moisture in the air to the image bearing member bearing an electrostatic latent image, to improve the photosensitivity of the image bearing member and to stabilize the surface potential of the image bearing member.
The heater heats the image bearing member to keep the image bearing member at a predetermined temperature not only during the image forming operation, but also during an inoperative condition of the image forming apparatus so that an image forming operation can be started at any time.
A developing device for developing the electrostatic latent image has a developing sleeve arranged in a confronting relation to the image bearing member. During the image forming operation, the developing sleeve is rotated to apply developer to the image bearing member; whereas, in the inoperative condition, the developing sleeve is stopped, in a position opposite the image bearing member.
As mentioned above, since the image bearing member is being heated by the heater even in the inoperative condition, only a surface of the stopped developing sleeve which is opposed to the image bearing member is subjected to radiant heat from the image bearing member. Consequently, since a portion of the surface of the developing sleeve which is opposed to the image bearing member is thermally expanded, and the remaining surface of the developing sleeve is not thermally expanded, as shown in FIG. 1B, the developing sleeve 15 is deformed to have slight deflection. (Note, the deflection of the developing sleeve is exaggerated in FIG. 1B to facilitate understanding of this feature). After the developing sleeve is rotated, since the whole peripheral surface of the developing sleeve is uniformly subjected to the radiant heat from the heater 21 in the image bearing member 3, the developing sleeve gradually returns to its original straight condition as shown in FIG. 1A.
If the developing sleeve starts to rotate from the deflected condition as shown in FIG. 1B, then a minimum distance X between the image bearing member 3 and the developing sleeve 15 will vary periodically. One period (cycle) corresponds to one revolution of the developing sleeve 15. If the distance X is periodically varied, an amount of the developer applied from the developing sleeve 15 to the image bearing member 3 will also be periodically changed, because the intensity of an electric field generated between the image bearing member and the developing sleeve by applying the developing bias voltage to the developing sleeve will be periodically varied in response to the variation of the distance X. In any case, the shorter the distance X, the greater the developer amount applied to the image bearing member 3, and the longer the distance X, the smaller the developer amount applied to the image bearing member. Therefore, a periodic density irregularity will occur in the developed image. Such density irregularity will be noticeable particularly in a developing device wherein the developer is transferred from a developing sleeve 15 to an image bearing member 3 by scattering the developer without contacting a layer of the developer carried on the developing sleeve 15 with the image bearing member 3.